This application is based on Japanese Patent Application No. 2001-241124 filed on Aug. 8, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an optical device for making light converge, and particularly to an optical device for producing a convergent light beam with a great numerical aperture for use in, for example, an optical system of a microscope or optical recording apparatus.
2. Description of the Prior Art
In an optical microscope that permits the observation of a test sample by the use of light, or an optical recording apparatus that permits the recording, reproducing, and erasing of information by the use of light, to achieve a high resolution or high recording density, it is essential to make light converge in a minute area on a test sample or recording medium. The size of the spot formed by a convergent light beam is inversely proportional to the numerical aperture (NA) of the light beam, and therefore, the greater the NA of a light beam is made, the smaller its spot diameter can be made.
The NA of a convergent light beam is given by equation (1) below, where n represents the refractive index through which the convergent light beam passes and xcex8max represents the maximum angle that the convergent light beam makes with its own optical axis (i.e., the angle between the outermost ray and the optical axis of the convergent light beam).
NA=nxc2x7sin(xcex8max)xe2x80x83xe2x80x83(1) 
Accordingly, effective ways to reduce the diameter of the spot formed by a convergent light beam is to increase the maximum angle xcex8max and to increase the refractive index n of the medium in addition. Immersion techniques used in microscopes depend on the latter, and achieve a greater NA by filling the space between an objective lens and a test sample with a liquid having a high refractive index. An immersion technique using oil as a high-refractive-index liquid is called oil immersion, one using water is called water immersion, and one using a solid instead of a liquid is called solid immersion.
Optical devices exploiting solid immersion are best exemplified by solid immersion lenses (SILs) and solid immersion mirrors (SIMs). A SIL is generally used in combination with another objective lens. FIG. 15 shows such a structure. The SIL 51 is hemispherical, and a convergent light beam emanating from an objective lens 52 enters the SIL 51 through a spherical surface 51a thereof and exits the SIL 51 through a flat surface 51b thereof. The SIL 51 and the objective lens 52 are so arranged that all the rays of the convergent light beam from the objective lens 52 are incident substantially perpendicularly on the spherical surface 51a. Thus, the light enters the SIL 51 without being refracted by the spherical surface 51a, and converges on the flat surface 51b. In this way, the SIL 51, although a lens, is used in such a way as not to exert any power that makes light converge.
A SIM is produced by forming a convex reflective surface on a surface of a base material, and on this reflective surface, which is a concave surface when seen from inside, light is shone from inside so as to be made to converge by reflection. Thus, the SIM, although a single device, has the functions of both the SIL 51 and the objective lens 52 described above. In addition, the SIM does not produce aberrations as are inevitable when light is made to converge by refraction, and thus more readily permits light to converge at one point on the optical axis. However, simply forming a convex reflective surface on a surface of a base material results in a reflective device, i.e., a device on which light is shone from the same direction in which the light is reflected by the device. This makes effective use of solid immersion difficult, and imposes severe constraints on the use of such a device.
A SIL that is used in such a way as to make light converge has also been proposed (Japanese Patent Application Laid-Open No. H11-45455). FIG. 16 shows this structure. Light enters the SIL 53 through an aspherical surface 53a thereof and exits the SIL 53 through a flat surface 53b thereof. The thickness of the SIL 53 (its dimension in the direction perpendicular to the flat surface 53b) is made equal to its focal length as a lens. Thus, the SIL 53 refracts a parallel light beam with the entrance surface 53a and thereby converges the light beam onto the flat surface 53b. 
This SIL 53 configured as described above is easy to use, because it does not need to be combined with another objective lens, nor does it require alignment of optical axes or adjustment of a distance. However, with this SIL 53, it is impossible to obtain a convergent light beam with a NA greater than a certain limit. For example, to form a parallel light beam into a convergent light beam with a NA of 1, if the base material is assumed to have a refractive index of 1.8, the angle of incidence of the outermost ray with respect to the entrance surface 53a needs to be 63.4xc2x0. This makes the fabrication of the SIL 53 very difficult.
With the advantages and disadvantages of both SILs and SIMs in mind, the inventors of the present invention have proposed a solid immersion device that makes light converge by both refraction and reflection (Japanese Patent Application Laid-Open No. 2000-162503). FIG. 17 shows this structure. This optical device 54, like a SIM, has a convex surface formed on a surface of a base material, but a reflective surface is formed only in a central portion of this convex surface 54a, with a peripheral portion thereof left as a transmissive surface. The surface of the base material opposite to the convex surface 54a is formed into a flat surface 54b. Light enters the optical device 54 through the peripheral portion of the convex surface 54a, and is thereby formed into a convergent light beam by refraction. The light is then reflected on the flat surface 54b so as to be directed to the central portion of the convex surface 54a, where the light is reflected again and is thereby formed into a more convergent light beam. Thus, the light is eventually made to converge on the flat surface 54b so as to exit the optical device 54 through the flat surface 54b. 
This optical device 54, although a transmissive device like a SIL, i.e., a device on which light is shone from the direction opposite to the direction in which the light exits the device, produces a convergent light beam with a great NA without an undue increase in the angles of incidence of rays with respect to the convex surface 54a. Moreover, this optical device 54 can be made thinner than a comparable SIL.
An optical device that makes light converge by diffraction has also been proposed (Japanese Patent Application Laid-Open No. H10-92002). This optical device has a diffraction grating formed on one surface of a base material shaped like a flat plate. Light enters the optical device through this diffraction grating, and is thereby formed into a convergent light beam, which exits the optical device through the opposite surface thereof.
An optical device that makes light converge by diffraction can be formed as a solid immersion device by giving it an appropriate thickness.
However, even when an optical device that makes light converge by diffraction is formed as a solid immersion device, it is still impossible to obtain a convergent light beam with a NA greater than a certain limit. To obtain a large angle of diffraction, the pitch of the diffraction grating needs to be reduced. However, reducing the pitch of the diffraction grating too much causes a phenomenon called anomaly, which extremely lowers diffraction efficiency. FIG. 18 shows an example of the relationship between the ratio of the pitch d of a diffraction grating to the wavelength xcex of light and diffraction efficiency. Diffraction efficiency drops abruptly starting from a d/xcex of about 1.7 down, which phenomenon is referred to as anomaly. The shape of a diffraction grating determines how high its diffraction efficiency is. However, the range of d/xcex in which the diffraction efficiency of a diffraction grating remains substantially constant does not depend on the shape of the diffraction grating. That is, irrespective of the shape of a diffraction grating, anomaly occurs at a d/xcex of about 1.7 or less.
Consider how a convergent light beam with as high a NA as possible can be obtained with a solid immersion device 55 shown in FIGS. 19A and 19B. FIGS. 19A and 19B are a plan view and a sectional view, respectively, of the optical device 55, which is composed of a base plate 56 having a refractive index of n and a diffraction grating 57 formed on a surface 56a thereof The diffraction grating 57 is formed concentrically, and, to permit light to converge at one point, its pitch d is so set as to decrease away from the center.
Now, suppose that light having a wavelength of xcex is perpendicularly incident on the diffraction grating 57, and that the outermost ray is diffracted at an angle of diffraction of ƒmax. Then, if the pitch of the diffraction grating 57 in a portion thereof where the outermost ray is incident is dext, equation (2) below holds. From equations (1) and (2), equation (3) below is derived.
dextxc2x7sin(xcex8max)=xcex/nxe2x80x83xe2x80x83(2) 
NA=xcex/dextxe2x80x83xe2x80x83(3) 
Here, attempting to make the NA equal to 1 is equivalent to attempting to make the pitch dext equal to the wavelength. However, as shown in FIG. 18, when d/xcex equals 1, diffraction efficiency is almost zero, and therefore no diffraction occurs in practical terms. That is, with the optical device 55 shown in FIGS. 19A and 19B, it is impossible to obtain a convergent light beam with a NA of 1.
The lower limit of d/xcex that yields satisfactorily high diffraction efficiency is about 1.7, and this determines the maximum NA obtained with a single diffraction grating. Equation (3) gives this maximum value of the NA as about 0.59. FIG. 20 schematically shows how the optical device 55 changes the NA. This figure shows a case in which the pitch dext is twice the wavelength xcex, and thus shows that the diffraction by the diffraction grating 57 causes the incident light to reach the opposite surface 56b as a convergent light beam with a NA of 0.5. In the figure, Lax represents the ray along the optical axis Ax, and Lext represents the outermost ray.
As discussed above, conventional optical devices do not readily produce a convergent light beam with a satisfactorily great NA, and thus have been becoming unable to keep up with demand for smaller beam spot diameters.
An object of the present invention is to provide an optical device that produces a convergent light beam with a great NA.
To achieve the above object, according to the present invention, an optical device for making the light shone into it converge is provided with a plurality of diffracting portions each making the light passing therethrough converge by diffraction, and the light shone into the optical device is passed through one after another of the plurality of diffracting portions so that the light is made to converge to a higher degree every time the light passes through one of the diffracting portions. This optical device makes light converge by diffraction, and achieves this not by diffracting light only once but by doing so a plurality of times on the plurality of diffracting portions so as to make the light converge to increasingly high degrees stepwise. Although there is an upper limit on the NA of a convergent light beam obtained with a single diffraction grating as described above, by making light converge to increasingly higher degrees stepwise, it is possible to obtain a convergent light beam with a NA greater than that upper limit.
Here, the diffracting portions may be of the type that makes light converge by transmitting the light, or of the type that makes light converge by reflecting the light. The diffracting portions may be formed on a surface of the optical device, or at an interface inside the optical device. Irrespective of whether formed on a surface or interface, the diffraction portions may be of the type that diffracts the light transmitted therethrough, or of the type that diffracts the light reflected therefrom. In a case where a diffracting portion is formed on a surface, it is also possible to form this diffracting portion as one that diffracts the light transmitted therethrough so that light is introduced into the optical device by being transmitted through this diffracting portion, that is, to form a diffracting portion on the entrance surface through which light enters the optical device.
It is also possible even to form two or more diffracting portions in different areas on a single surface. Directing light from one to the other of two diffracting portions formed in different areas on a single surface can be achieved easily by providing a reflective surface, for example by the use of another diffracting portion. The single surface may be a surface of the optical device, or an interface inside the optical device. An interface inside the optical device denotes an interface between two media having different optical properties, and, as long as such a surface is continuous and has the same medium throughout on one side, it is regarded as a single surface irrespective of whether flat or curved.
The diffracting portions may be diffraction gratings. By forming the optical device out of two or more base materials having different refractive indices, it is possible to form a diffraction grating inside the optical device.
It is advisable to make the light shone into the optical device converge in such a way that the light has the minimum diameter on the surface through which the light exits the optical device. This permits the optical device to function as a solid immersion device. That is, the convergent light beam with a great NA obtained through the plurality of diffraction portions can be shone directly on a target. In this way, it is possible to obtain an extremely small spot.
It is also possible to provide a light-shielding member on the surface through which the light shone into the optical device exits the optical device, with an opening smaller than the diameter of the light on the surface formed in the light-shielding member, so that the light is made to exit the optical device through the opening. This permits only a central portion of the convergent light beam to be shone on a target. In this way, it is possible to obtain a spot even smaller than that formed by the convergent light beam with an increased NA obtained through the plurality of diffracting portions.
It is also possible to form part of a surface of the optical device into a protruding portion so that the light shone into the optical device is made to exit the optical device through the protruding portion. The optical device, which produces a convergent light beam with a great NA, is used in the immediate vicinity of a target, and therefore may make contact with the target if inclined even slightly. By making only a portion of the surface of the optical device facing the target protrude, it is possible to greatly reduce the risk of such contact.